A number of factors combine each year to determine the U.S. average corn yield.  Among those factors, temperature and precipitation during July are the most important.  Crop yield models have long confirmed the large yield impact of July weather.  The most favorable weather conditions in July in the heart of the corn belt consist of temperatures that are modestly below average and precipitation that is about 25 percent above average.  These are the kind of conditions that were experienced in 2009 and contributed to the record high U.S. average yield that year.  Historically, such conditions over large areas have been rare.